Printed circuit boards comprise a board of plastics, for example, epoxy resin, which is copper lined on both sides and into which the tracks and connecting places are etched. Thin plastics coatings are applied by various known process for the insulation of the printed circuit boards. The prior art lacquering process include application by a squeegee, immersion, spraying and also electrostatic coating. The electrostatic lacquering of printed circuit boards is described in detail in German OS No. 1 772 976. High voltage is generated between a spraying head and the printed circuit board to be lacquered, the board being applied to ground potential. Due to the high electric potential between the rotating spraying head and the printed circuit board, lacquer droplets are conveyed to the printed circuit board surface adjacent the spraying head and melt on such surface to produce a lacquer film.
In the practical operation of electrostatic lacquer coating installations of the kind specified the printed circuit boards were moved lying flat and horizontal on paper past the lacquering station. After the lacquer layer had dried, the printed circuit board was turned and its major surface coated in the same manner, the major surface then also having to be dried. A disadvantage of that process is that it is expensive in time and money to perform lacquering and drying twice. Another disadvantage is that the lacquer layer applied first must be dried twice, namely for the first time when it is itself dried, and for the second time when the lacquer layer of the opposite surface is dried. As a result there is a different consistency in the two lacquered layers on the two surfaces of the printed circuit boards. The lacquer layer first applied may even become damaged during the drying of the second lacquer layer, so that the printed circuit board becomes unusable.
German OS No. 33 04 648 discloses a process for the electrostatic application of a lacquer film to magnetic tapes, wherein the lacquer dispersion is applied electrostatically to both surfaces of the tape simultaneously, whereafter the two lacquer layers are dried simultaneously. However, this prior art process, which uses oppositely disposed spraying nozzles, can be applied only to the coating of continuous tapes; during the coating of items such as printed circuit boards, guided at intervals past the spraying nozzles, in the intervals between successive items to be coated the sprayed streams from the opposite nozzles would impinge on one another and eddy uncontrollably, such disturbances preventing the obtaining of a clearly-defined layer thickness. It also calls for a relatively large amount of space to dispose the spraying nozzles on both sides of the tape, as in the case of the process disclosed in German OS No. 33 304 648.